1. Field of the Invention
The invention relates generally to a circuit configuration and method of manufacturing a transient voltage suppressor (TVS). More particularly, this invention relates to an improved circuit configuration and method of manufacturing vertical TVS array implemented with trench isolation for resolving a technical difficulty of latch-up.
2. Description of the Relevant Art
The conventional technologies for designing and manufacturing a transient voltage suppressor (TVS) array is still confronted with a technical difficulty that the in a TVS array wherein multiple PN junctions diodes are manufactured in a semiconductor substrate by applying a standard COMS processing steps, there are inherent PNP and NPN parasitic transistors. In an ESD event or the occurrence of a transient voltage, with a larger voltage applied to this TVS array, the parasitic NPN or PNP transistors are turned on and latched up, thus causing a sudden and strong voltage snap back. The sudden and large snapback may cause the undesirable effects of system instability or even damages. Additionally, the latch-up of the parasitic NPN or PNP transistors in the TVS array may further lead to other unexpected or undesirable voltage-current transient conditions. The technical difficulties caused by the parasitic PNP or NPN latch-up in the TVS array cannot be easily resolved.
Specifically, the transient voltage suppressors (TVS) are commonly applied for protecting integrated circuits from damages due to the inadvertent occurrence of an over voltage imposed onto the integrated circuit. An integrated circuit is designed to operate over a normal range of voltages. However, in situations such as electrostatic discharge (ESD), electrical fast transients and lightning, an unexpected and an uncontrollable high voltage may accidentally strike onto the circuit. The TVS devices are required to serve the protection functions to circumvent the damages that are likely to occur to the integrated circuits when such over voltage conditions occur. As increasing number of devices are implemented with the integrated circuits that are vulnerable to over voltage damages, demands for TVS protection are also increased. Exemplary applications of TVS can be found in the USB power and data line protection, Digital video interface, high speed Ethernet, Notebook computers, monitors and flat panel displays.
FIGS. 1A and 1B show a circuit diagram and a current-voltage diagram respectively of a TVS device. An ideal TVS is to totally block the current, i.e., zero current, when the input voltage Vin is less than the breakdown voltage Vb for minimizing the leakage current. And, ideally, the TVS has close to zero resistance under the circumstance when the input voltage Vin is greater than the breakdown voltage Vb such that the transient voltage can be effectively clamped. A TVS can be implemented with the PN junction device that has a breakdown voltage to allow current conduction when a transient input voltage exceeds the breakdown voltage to achieve the transient voltage protection. However, the PN junction type of TVS has no minority carriers and has a poor clamping performance due to its high resistance as that shown in FIG. 1B. There are alternate TVS implementations with Bipolar NPN/PNP with an avalanche-triggered turning-on of the bipolar transistor. The base is flooded with minority carriers and the bipolar TVS can achieve better clamping voltage as the avalanche current is amplified with the bipolar gain.
With the advancement of electronic technologies, there are increasingly more devices and applications that require TVS diode array for ESD protection, particularly for protecting high bandwidth data buses. Referring to FIG. 2A for a circuit diagram of a four channel TVS and FIG. 2B for side cross sectional views of device implementation of the TVS array showing only the core of the array device. The TVS array as shown in FIGS. 2A and 2B includes a plurality of high-side and low-side steering diodes connect in series wherein the high-side steering diodes are connected to Vcc and the low-side steering diodes connected to ground potential. Furthermore, these high-side and low-side steering diodes are connected in parallel to a main Zener diode wherein the steering diodes are much smaller and having lower junction capacitance. Additionally, as shown in FIG. 2C, such implementation further generates another problem of latch-up due to the SCR action induced by parasitic PNP and NPN transistors. The main Zener diode breakdown triggers the NPN on which further turns on the SCR resulting latch-up. In high temperature, the high leakage current through the NP junction of the parasitic NPN may also turn on the SCR leading to latch-up even though the NPN is not turned on. To suppress latch-up due to the SCR action induced by parasitic PNP and NPN transistors, the actual device implementation on a semiconductor substrate requires a lateral extension on the substrate of a distance that may be up to 100 micrometers or more as shown in FIG. 2B and the suppression usually is not effective enough.
FIGS. 3A and 3B illustrate particular difficulty caused by latch-up through the parasitic PNP transistor in an Ethernet differential protection circuit. In this Ethernet protection circuit, both Vcc and ground pins are floating. However, a parasitic SCR structure is not sufficiently weak in the design that causes a sudden voltage snap back as shown in FIG. 3B. Such sudden and strong snap back may cause undesirable effects of system instability or even damages. The difficulties cannot be easily resolved because the parasitic PNP transistor is inherent in the standard CMOS process and the fact that both Vcc and ground pin floating deteriorates the effect of latch-up. Additional buried layers are required to suppress the gain of the parasitic PNP transistors that causes complicated device configurations and high manufacturing costs.
Therefore, a need still exists in the fields of circuit design and device manufactures for providing a new and improved circuit configuration and manufacturing method to resolve the above-discussed difficulties. Specifically, a need still exists to provide new and improved TVS circuits that can effectively and conveniently prevent the parasitic PNP/NPN transistor latch-up.